The present invention relates to a process for the preparation of organosilicon compounds (P) containing xcex1,xcex2-unsaturated carboxylic acid radicals, to polymers of the organosilicon compounds (P), to compositions comprising organosilicon compounds (P), and to optically anisotropic layers produced by alignment and polymerization of liquid-crystalline organosilicon compounds (P).
The syntheses and applications of crosslinkable organosiloxanes and organosilanes, in particular siloxanes and silanes containing methacryloyl groups, are described in large number in the literature. Frequently used materials, which are, for example, employed for coatings, are alkoxy-substituted silanes, which, besides the methacryloyl groups, usually also contain methoxy or ethoxy groups. Owing to the relatively low reactivity of the methacryloyl groups, however, the crosslinking density of the polymerized layers produced from these compounds is comparatively low. In many applications, an increase in the crosslinking density could produce better material properties, for example an improvement in the solvent stability, an improvement in the adhesion to the surfaces to be coated, or an increase in the hardness of the coatings. Possible solutions to the preparation of highly crosslinked structures of this type are to increase the number of polymerizable groups, and to use polymerizable groups of higher reactivity than that of the methacryloyl groups, for example acryloyl groups. However, the processes disclosed hitherto for the preparation of such organosiloxanes and organosilanes containing polymerizable groups of high reactivity can only be carried out with difficulty on an industrial scale, or do not give the desired result for reasons associated with the method of production. This is particularly true in the case of acryloyl groups.
It is known that crosslinked organosiloxanes having a liquid-crystalline structure are frequently insufficiently stable to external influences, such as exposure to certain organic solvents. U.S. Pat. No. 5,362,315 discloses, for example, pigments comprising liquid crystalline substances having a chiral phase which are distinguished by the fact that their color depends on the viewing angle. These pigments are employed in various transparent media, such as coatings, binders or plastics. However, EP-A-724005 discloses that when these pigments are prepared from liquid-crystalline organosiloxanes in which the polymerizable groups are exclusively methacryloyl groups, they then, depending on the processing conditions and the medium into which the pigments are incorporated, exhibit color changes which cannot be tolerated in a large number of applications. A solution to this problem, or at least a reduction in its magnitude, can likewise be achieved by increasing the network density of the aligned and crosslinked liquid-crystalline structure.
Various methods are known for the preparation of organosiloxanes and silanes containing methacryloyl groups. A process which is frequently used on an industrial scale comprises the hydrosilylation of di-unsaturated compounds containing double or triple bonds of different reactivity. The aim in this process is for an xcfx89-olefinically unsaturated group to be the target of Sixe2x80x94H attack, while the second unsaturated group is not hydrosilylated. To this end, the reactivity of the group which is not to be hydrosilylated must be lower than the reactivity of the other unsaturated groups. The unsaturated group of lower reactivity is preferably the methacryloyl double bond, but, in principle, the methacryloyl double bond can also be hydrosilylated. In general, more than 10% of side-reactions of methacryloyl groups with Sixe2x80x94H groups take place, the proportion of these side-reactions corresponding to the concentration of the methacryloyl double bonds. In general, the competing hydrosilylation of the two different unsaturated systems thus necessitates that undesired byproducts, for example, dimers, are always produced in such processes, in a proportion which generally depends on the nature of the unsaturated groups and on the manner in which the reaction is carried out.
If the polymerizable groups to be used are xcex1,xcex2-unsaturated carboxylic acid radicals of relatively high reactivity, such as, for example, the acryloyl double bond, the competition with xcfx89-olefinically unsaturated groups is significantly higher than in the case of methacryloyl double bonds under the conditions of the hydrosilylation reaction. Organosiloxanes or silanes containing acryloyl groups are therefore not readily accessible in the manner described above, since the high proportion of side-reactions results in partial crosslinking even during the hydrosilylation reaction, or in the case of siloxanes or silanes containing only one hydrogen atom bonded directly to silicon, in double addition of the siloxane or silane moiety to the di-unsaturated compound. If the organosiloxanes contain mesogenic side groups, the consequent increase in the viscosity usually reduces the mobility of the mesogens so much that a uniformly aligned liquid-crystalline phase can form only with difficulty, if at all.
U.S. Pat. No. 5,211,877 therefore describes, as an alternative method for the preparation of liquid-crystalline organosiloxanes or silanes containing methacryloyl or acryloyl groups, a multistep synthesis in which the methacryloyl or acryloyl group is introduced subsequently, by esterification using a reactive methacryloyl or acryloyl compound after hydrosilylation of a precursor containing a hydroxyl group protected by a protecting group, and subsequent removal of the protecting group. Owing to the large number of reaction steps necessary, however, this method tends to be more practicable for small laboratory syntheses. It is unsuitable for the production of highly crosslinkable organosiloxanes and organosilanes on an industrial scale.
An object of the present invention is to provide a process which can be implemented on an industrial scale for the preparation of crosslinkable organosilicon compounds containing xcex1,xcex2-unsaturated carboxylic acid radicals in high selectivity.
The invention relates to a process for the preparation of organosilicon compounds (P) containing xcex1,xcex2-unsaturated carboxylic acid radicals, of the general formula (1)
xe2x80x94Axe2x80x94Oxe2x80x94C(O)xe2x80x94CRxe2x95x90CH2xe2x80x83xe2x80x83(1),
in which, in a first step,
organosilicon compounds (H) containing hydrogen atoms bonded directly to silicon are reacted with olefinically unsaturated compounds (U) containing a terminal double or triple bond, of the general formula (2)
xcexa9-Oxe2x80x94C(O)xe2x80x94CRHxe2x80x94CH2xe2x80x94Zxe2x80x83xe2x80x83(2),
in the presence of metals or compounds from the platinum group as catalyst, to give organosilicon compounds (E) containing radicals of the general formula (3)
xe2x80x94Axe2x80x94Oxe2x80x94C(O)xe2x80x94CRHxe2x80x94CH2xe2x80x94Zxe2x80x83xe2x80x83(3),
and, in a second step, Hxe2x80x94Z compounds are eliminated from organosilicon compounds (E), where
A is a divalent organic radical,
xcexa9 is a monovalent organic radical containing a terminal double or triple bond,
R is an H atom or a methyl radical, and
Z is Cl, I, Br or 4-methyltoluenesulfonyl.
The process proceeds in high selectivity in both steps and therefore gives very pure organosilicon compounds (P) since the undesired hydrosilylation of the xcex1,xcex2-unsaturated radical xe2x80x94CRxe2x95x90CH2 in the general formula (1), and premature polymerizations caused thereby, are avoided.
The elimination of the Hxe2x80x94Z compounds is preferably carried out by means of a base, such as a tertiary amine, for example triethylamine or tributylamine, or a basic metal salt of an acid, such as, for example, K2CO3, Na2CO3, KHCO3, NaHCO3, Na acetate and KOC(O)C(CH3)xe2x95x90CH2. The Hxe2x80x94Z compound is then chemically bound to the base as a salt.
The organosilicon compounds (H) employed are, in particular, organosiloxanes, which may be linear, branched, or crosslinked organosiloxanes, or which may be in the form of organosilsesquioxanes, or organosilanes.
The organosiloxanes (H) employed are preferably built up from at least 2 identical or different units of the general formula (4)
[HpRlqSiO(4-p-q)/2]xe2x80x83xe2x80x83(4),
in which
R1 is a C1- to C10-alkyl or phenyl radical which is unsubstituted or substituted by halogen atoms, and
p and q each have the value 0, 1, 2 or 3,
where the sum of p and q is at most 3, and in at least one unit per molecule, p has the value 1, 2 or 3.
The organosiloxanes (H) are preferably built up from 2 to 30 units, in particular 2 to 15 units, of the general formula (4). The subscripts p and q preferably each have the value 1 in at least 30% of all units of the general formula (4). Preferred radicals R1 are methyl radicals. Particularly preferred siloxanes of the general formula (4) are cyclotetrasiloxanes, cyclopentasiloxanes, tetra-methyldisiloxanes and linear polymethylsiloxanes preferably having from 4 to 15 silicon atoms and trimethylsilyl groups as end groups.
The organosilanes (H) employed preferably have the general formula (5)
HsSiR2txe2x80x83xe2x80x83(5),
in which
R2 is a halogen atom or a C1- to C10-alkyl or phenyl radical which is unsubstituted or substituted by halogen atoms,
s has the value 1, 2, 3 or 4, and
t has the value 0, 1, 2, 3 or 4,
where the sum of s and t is at most 4.
The subscript s preferably has the value 1 or 2. Particularly preferred silanes of the general formula (5) are those which contain radicals R2 which are either all identical and are each a chlorine atom, or are different and are a combination of one or two halogen atoms and C1-C4-alkyl radicals or phenyl radicals.
Preference is given to compounds (U) in which, in the general formula (2), xcexa9 is R3xe2x80x94A0, where R3 is a monovalent radical of the formula CH2 xe2x95x90CHxe2x80x94(CH2)n or HCxe2x89xa1Cxe2x80x94(CH2)n, in which n is an integer having a value of from 0 to 8, and in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals, and A0 is a chemical bond or a divalent organic radical.
A0 can be prepared by known processes of synthetic organic chemistry. A hydrosilylatable group can be bonded to A0 by known chemical reactions, such as esterification, condensation, etherification, alkylation, alkenylation, alkynylation or acylation. A0 may additionally be capable of forming an ester bond by virtue of the organic radical A0 being bonded to the divalent oxygen atom of an ester carbonyloxy group.
In a preferred embodiment, the first process step is carried out using compounds (U) in which A0 is (CRH)mxe2x80x94, where m is an integer having a value of from 0 to 12, and R is an H atom or a methyl radical, and where one or more non-adjacent methylene units may be replaced by oxygen atoms, dimethylsilyl radicals, 1,4-substituted phenylene, or cyclohexylene units. The organosiloxanes (P) prepared therefrom are particularly suitable for the production of highly crosslinked coatings and as additives for surface-coating preparations.
In order to prepare liquid-crystalline organosiloxanes (P), mesogenic compounds are hydrosilylated onto the organosilicon compounds (H) in this process.
In a preferred embodiment, polymerizable liquid-crystalline organosiloxanes (P) are prepared. To this end, use may be made, in a first process step, of mesogenic compounds, preferably selected from compounds of the general formula (6)
R3xe2x80x94X1xe2x80x94(A1xe2x80x94X2)dxe2x80x94R5xe2x80x94Oxe2x80x94C(O)xe2x80x94CH(R)xe2x80x94CH2xe2x80x94Zxe2x80x83xe2x80x83(6)
and compounds of the general formula (7)
R3xe2x80x94X1xe2x80x94(A1xe2x80x94X2)dxe2x80x94R5xe2x80x94A2xe2x80x83xe2x80x83(7),
where
R3, R and Z are as defined above,
R5 is a chemical bond or a radical of the formula (CH2)m, in which m is an integer having a value of from 1 to 12, and in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals,
X1 is selected from a chemical bond and the divalent radicals xe2x80x94Oxe2x80x94, xe2x80x94C(O)Oxe2x80x94 and xe2x80x94OC(O)xe2x80x94,
X2 is a binding member selected from a chemical bond and the divalent radicals xe2x80x94C(O)Oxe2x80x94, xe2x80x94OC(O)xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94, xe2x80x94Nxe2x95x90CHxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94, xe2x80x94C(O)NHxe2x80x94, xe2x80x94NHC(O)xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Nxe2x95x90N(O)xe2x80x94 and xe2x80x94N(O)xe2x95x90Nxe2x80x94,
A1 is a divalent radical selected from six-membered homocyclic or heterocyclic rings, such as 1,4-phenylene, 1,4-cyclohexylene, 2,5-pyridinylene, 2,5-pyranylene, 2,5-pyrimidinylene, 5,2-pyrimidinylene, 2,5-(1,3-dioxanylene) and 5,2-(1,3-dioxanylene), which are unsubstituted or substituted by cyano, fluorine or methyl groups, or from bicyclic compounds consisting of six-membered rings, such as 2,6-naphthylidene, 2,7-naphthylidene and 1,4-naphthylidene,
A2 is an end group selected from hydrogen, halogen, hydroxyl, nitrile, methacryloyloxy, methacryloylethyleneoxy, cholestane, cholesteryl, doristeryl, monofunctional dianhydrohexitol, cyclohexyl and alkenyl radicals having 1 to 10 carbon atoms, in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals, and
d can have the value 2 or 3.
Preferably, at least 5% of the radicals of the organosiloxanes (P) are prepared by hydrosilylation using compounds selected from mesogenic compounds of the general formula (6) and compounds of the general formula (2) in which A0 is a chemical bond or a divalent organic radical of the above formula (CRH)mxe2x80x94.
Preferably, at least 20% of the radicals of the organosiloxanes (P) are mesogenic radicals. The term xe2x80x9cmesogenic radicalsxe2x80x9d is taken to mean groups which can produce liquid-crystalline properties in a molecule. A regularly updated collection of known mesogenic groups is published by V. Vill et al. as a database entitled LiqCryst (can be purchased from LCI Publisher GmbH, Eichenstr. 3, D-20259 Hamburg).
It has been found that the introduction of the xe2x80x94C(O)Oxe2x80x94 or xe2x80x94OC(O)xe2x80x94 groups as radicals X2 favorably affects the formation of homogeneous liquid-crystalline phases. These radicals are therefore preferred.
Preferred mesogenic compounds (U) of the general formula (6) are the compounds (U) of the general formula (8) 
where X2, R5 and Z are as defined for the general formula (6).
In the particularly preferred compounds (U) of the general formula (8), X2 are selected from a chemical bond and divalent radicals from the group consisting of xe2x80x94C(O)Oxe2x80x94 and xe2x80x94OC(O)xe2x80x94, R5 are selected from a chemical bond and C1- to C6-alkyl radicals, and Z is a chlorine atom. R3 is preferably a CH2 xe2x95x90CHxe2x80x94CH2xe2x80x94 group.
In the hydrosilylation, the organosiloxanes (H) containing hydrogen atoms bonded directly to silicon, which are preferably built up from units of the general formula (4) or (5), are reacted simultaneously or successively with a freely adjustable mixture of olefinically unsaturated compounds (U) containing terminal double or triple bonds, of the general formula (2), in the presence of, preferably, at least one metal from the platinum group and/or compounds thereof, where the total number of moles of the olefinically unsaturated compounds (U) corresponds to the total number of Sixe2x80x94H bonds in the organosiloxanes (H), or, in order to ensure complete saturation, an excess of one of the two components (H) and (U) of up to 20% is initially introduced. Suitable hydrosilylation processes are described, for example, in U.S. Pat. No. 5,211,877, U.S. Pat. No. 5,214,077 and DE-A-19541838.
Examples of metals from the platinum group and/or compounds thereofxe2x80x94referred to below as platinum catalystxe2x80x94which can be employed in the process according to the invention are platinum, palladium, rhodium, iridium and compounds thereof, preferably platinum and/or compounds thereof. It is possible to employ all catalysts which have also been employed hitherto for the addition reaction of hydrogen atoms bonded directly to Si atoms onto aliphatically unsaturated compounds. Examples of such catalysts are metallic and finely divided platinum, which can also be on supports such as silicon dioxide, aluminum oxide or activated carbon, compounds or complexes of platinum such as platinum halides, for example PtCl4, H2PtCl6.6 H2O, Na2PtCl4.4 H2O; platinum-olefin complexes, platinum-alcohol complexes, platinum-alkoxide complexes, platinum-ether complexes, platinum-aldehyde complexes, platinum-ketone complexes, including products of the reaction of H2PtCl6.6H2O and cyclohexanone, platinum-vinylsiloxane complexes, in particular platinum-divinyltetramethyldisiloxane complexes with or without a content of detectable inorganically bound halogen, bis(gamma-picolinyl)platinum dichloride, trimethylenedipyridinylplatinum dichloride, dicyclopentadienylplatinum dichloride, (dimethyl sulfoxide)ethyleneplatinum(II) dichloride and products of the reaction of platinum tetrachloride with an olefin and a primary amine, a secondary amine, or both a primary amine and a secondary amine, such as the product of the reaction of platinum tetrachloride dissolved in 1-octene with sec-butylamine, or ammonium-platinum complexes as described in EP-B 110 370.
The platinum catalyst is preferably employed in amounts of from 0.05 mmol to 0.50 mmol, based on the amount of elemental platinum or the platinum compounds used per mole of the Si""H groups present in the siloxane employed. The reaction is preferably carried out at temperatures of from 0xc2x0 C. to 110xc2x0 C. and preferably at pressures of from 0.05 MPa to 1.0 MPa.
The hydrosilylation can be carried out in the presence or absence of solvents, such as hydrocarbons, ethers or esters. If the reaction is carried out in a solvent or solvent mixture, aprotic solvents or solvent mixtures having a boiling point of up to 160xc2x0 C. at about 0.1 MPa are preferred. The individual reactants need not necessarily be soluble in the solvent, since the reaction can also be carried out in a suspension or emulsion. The reaction can also be carried out in a solvent mixture having a miscibility gap if at least one of the reactants is soluble in each of the two phases.
An advantage of this hydrosilylation process is that work-up of the reaction product containing organosilicon compounds (E) is not necessary before the elimination of the Hxe2x80x94Z compounds in the second process step. After completion of the elimination reaction and purification of the organosilanes (P) formed as reaction product, these organosilanes (P) containing hydrolyzable radicals R2 and preferably prepared from silanes of the general formula (5) can be equilibrated and condensed by known processes, as described, for example, in U.S. Pat. No. 5,214,077, to form organosiloxanes (P). In particular, the elimination process can also be used for the production of organosiloxanes (P) having a liquid-crystalline phase.
The organosilicon compounds (P) prepared by the elimination process can be used, for example, for the preparation of highly crosslinked coatings by polymerization, and if these compounds have liquid-crystalline phases, aligned and crosslinked layers having a liquid-crystalline structure can be produced and prepared for various applications. It is also possible to prepare compositions which comprise organosilicon compounds (P) which can be crosslinked to give polymers.
The elimination process enables the production of organosiloxanes and organosilanes (P) having a high concentration of xcex1,xcex2-unsaturated carboxylic acid radicals of high reactivity, such as, for example, acryloyl groups. In the polymerization of these compounds, a high crosslinking density is achieved. The coatings produced therefrom are particularly distinguished by increased hardness, improved scratch resistance and increased resistance to organic solvents.
The liquid-crystalline organosilanes and organosiloxanes (P) prepared by the elimination process can be used for the preparation of highly crosslinked, liquid-crystalline polymers, for which various possible applications are known from the prior art. Owing to their optically anisotropic properties, they are particularly suitable, for example, for the production of optically anisotropic layers, for example optical retardation films, interference pigments and wavelength- and polarization-selective optical filters.
The production of optically anisotropic layers of this type generally requires a uniform alignment of the mesogens in the shortest possible time after application of the layer. Such additional requirements, such as, for example, rapid alignment, which is favored by a low viscosity of the crosslinkable liquid-crystalline polymer, can be achieved more easily by admixing suitable low-molecular-weight components. The organosiloxanes (P) prepared by the elimination process are therefore also used in mixtures with other liquid-crystalline or non-liquid-crystalline materials so long as these additional mixture components do not prevent the formation of the liquid-crystalline phase.
The preferred additional mixture components used are compounds of the general formula (6) and compounds of the general formulae (9) and (10)
R7xe2x80x94X1xe2x80x94(A1xe2x80x94X2)dxe2x80x94R5xe2x80x94A3xe2x80x83xe2x80x83(9),
A4xe2x80x94R5xe2x80x94(X2xe2x80x94A1)dxe2x80x94X1xe2x80x94R5xe2x80x94Kxe2x80x94R5xe2x80x94X1xe2x80x94(A1xe2x80x94X2)axe2x80x94R5xe2x80x94A4xe2x80x83xe2x80x83(10),
where
R5, X1, X2, A1 and d, independently, can be the same or can be different from one another and are as defined for the general formulae (6) and (7),
R7 is selected from the group consisting of acryloyloxy, methacryloyloxy and acryloyl- and methacryloylethyleneoxy radicals and the group of radicals having the formula H2Cxe2x95x90CHxe2x80x94(CjH2j-l), in which j is an integer having a value of from 1 to 8, and in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals,
A3 is selected from the group consisting of hydrogen atoms, halogen atoms, hydroxyl, nitrile, acryloyloxy, methacryloyloxy, acryl- and methacryloxyethyleneoxy radicals, cholestane radicals, cholesteryl radicals, doristeryl radicals, dianhydrohexitol radicals, cyclohexane radicals and alkenyl radicals having 1 to 10 carbon atoms, in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals
K is selected from the group consisting of dianhydrohexitol derivatives and in particular dianhydrosorbide or dianhydromannitol,
A4 are identical or different radicals selected from the group consisting of hydrogen atoms, halogen atoms, hydroxyl, nitrile, acryloyloxy, methacryloyloxy, acryl- and methacryloxyethyleneoxy radicals, cyclohexane radicals and alkenyl radicals having 1 to 10 carbon atoms, in which one or more non-adjacent methylene units may be replaced by oxygen atoms or dimethylsilyl radicals.
The invention likewise relates to optically anisotropic layers comprising liquid-crystalline organosiloxanes (P) prepared by the elimination process, or mixtures thereof, both with one another and with other liquid-crystalline or non-liquid-crystalline materials, so long as these additional mixture components do not prevent the formation of a liquid-crystalline phase.
The optically anisotropic layers are preferably produced by a process in which liquid-crystalline organosiloxanes (P) or mixtures containing organo-siloxanes (P) are applied to a substrate, aligned and subsequently fixed by a chemical reaction. The optically anisotropic layers are most preferably produced using liquid-crystalline organosiloxanes (P) prepared from compounds of the general formulae (4), (5), (6) and (7) and mixture components of the general formulae (6), (9) and (10).
The liquid-crystalline organosiloxanes (P) or an LC mixture containing the liquid-crystalline organosiloxanes (P) can be applied to the substrate surface in solution or as a dry substance at above the glass transition temperature of the solvent-free dry substance, for example by spin coating or using a knife coater or roller. If a solvent is used for the application, this must be removed in a subsequent drying step.
The thickness of the dry LC layer on the substrate depends on the requirements of the particular application. If the layer is used, for example, as a retardation plate, the necessary thickness is then the quotient of the optical retardation required and the optical anisotropy of the aligned LC layer. The thickness of the dry LC layer is preferably between 1 xcexcm and 500 xcexcm, particularly preferably between 1 xcexcm and 60 xcexcm,
The application and alignment of the LC mixture can be carried out fully continuously, semi-continuously or discontinuously. A fully continuous process is described in U.S. Pat. No. 5,362,315.
The LC layer can be covered by a second substrate. The mesogens are aligned, for example, by shearing the material during application or, for example, after application through the interaction of the mesogens with the appropriately selected substrate surface(s) or by means of an electric field.
The LC mixture is preferably aligned in a temperature range from above the glass transition temperature to below the commencement of clearing of the particular LC mixture. In order to facilitate a simple industrial process, the composition of the LC mixture is preferably adjusted so that the suitable alignment temperature is between 20xc2x0 C. and 150xc2x0 C.
If the alignment of the mesogens is to take place through an interaction with the substrate surface(s), a suitable alignment layer can, in order to improve the aligning action, be applied to the substrate surface(s) by known coating, printing or dipping processes described in large number in the literature. The alignment layers or the substrates can be provided with a surface structure which favors alignment through additional treatment, for example rubbing. A local change in the alignment direction is possible, for example, by known methods for structuring the alignment layer by means of exposure to polarized UV light. Suitable methods for achieving a tilt between the mesogens of a liquid-crystalline phase and their interfaces are likewise described in the literature, for example the vapor deposition of inorganic materials at an oblique angle. In order to achieve a tilt of the mesogens at an angle of from 10xc2x0 to 80xc2x0 relative to the substrate surface, a layer of silicon oxide is particularly preferably applied by vapor deposition.
Substrates which can be used are all materials which are known for the production of optical elements. Preference is given to organic and inorganic substrates which are transparent or semi-transparent in the wavelength range relevant for the particular application. The substrates can be planar or curved. Particular preference is given to substrates which do not change their physical properties at the production, processing and use temperature of the LC layers.
Very particular preference is given to glass and quartz plates and polymer films, such as, for example, polycarbonates, polysulfones, polyethylene terephthalates, polyimides and cellulose acetates. If necessary, the substrate(s) can be provided with an additional alignment aid, such as, for example, a layer of polyimide, polyamide, polyvinyl alcohol or silicon oxide.
When the alignment is complete, the liquid-crystalline organosiloxanes (P) or the LC mixtures containing these liquid-crystalline organosiloxanes (P) are fixed in the optically anisotropic layers. To this end, the organosiloxanes (P) are crosslinked via the xcex1,xcex2-unsaturated carboxylic acid radicals present in the mesogenic radicals. This crosslinking is preferably effected by means of free radicals generated by peroxides or other suitable thermally activatable free-radical formers, by UV light, by high-energy electromagnetic radiation, or by warming. However, the crosslinking can also be effected by means of crosslinking agents containing hydrogen atoms bonded directly to silicon with catalysis by platinum metal catalysts. It can also take place cationically or anionically. Particular preference is given to the UV light crosslinking described in U.S. Pat. No. 5,211,877 and U.S. Pat. No. 5,214,077.
The resultant fixed layer can be used together with the substrate in the form of a laminate, as a film open on one side, or, after removal of the substrate(s), also as a free film. Preference is given to the use as a film together with the substrate or as a film open on one side.
Another use form of the optically anisotropic layers is as optically anisotropic platelets, which are also referred to hereinbelow as LC platelets. U.S. Pat. No. 5,362,315 discloses how pigments having a liquid-crystalline structure with a chiral phase which reflect light in colors can be prepared by detaching a polymerized cholesteric film from the substrate and subsequently comminuting the rough fragments obtained in this way. The pigments can then be incorporated into a suitable binder system and applied to a substrate. DE-A-196 19 460 describes how platelets having a negative refractive index anisotropy for visible light can be prepared and used by a similar process. The layers described above can likewise be comminuted after crosslinking to give optically anisotropic platelets and subsequently incorporated into a binder and applied to a substrate.
The LC platelets are most preferably produced using liquid-crystalline organosiloxanes and organosilanes (P) prepared by the elimination process from compounds of the general formulae (4), (5), (6) and (7) and mixture components of the general formulae (6), (9) and (10).
Coherent films of the layers of liquid-crystalline organosiloxanes (P) can be employed for all purposes for which the optically anisotropic layers of positive and negative refractive index anisotropy are suitable, for example as optical retarder films for improving the properties of liquid-crystal displays, which are described in large number in the literature. Depending on the choice of substrates and alignment layers and the composition of the liquid-crystalline organosiloxanes (P) and the LC mixtures containing these organosiloxanes (P), it is possible to achieve different forms of alignment, which can advantageously be employed, for example, in liquid-crystal displays, such as TN or STN displays. Examples of possible alignments of the mesogens in the layers are a homogeneous and planar alignment of all mesogens, a hybrid alignment in which the alignment changes continuously from planar to homeotropic from one surface to the opposite surface, a completely homeotropic alignment of all mesogens, a planar alignment which is twisted about the surface perpendiculars, in which the mesogens are aligned, for example, by doping with a chiralic or by means of mutually twisted alignment layers, in a similar manner to in a TN or STN cell, or a cholesteric alignment with a pitch which is less than the wavelength of visible light, which, as described in DE-A-196 19 460, results in a negative refractive index anisotropy.
Further applications can be accomplished through slight modification of the above-described process for the production of the optically anisotropic layers. For example, absorptive polarizing filters can be produced if a mixture is used which, in addition to the liquid-crystalline organosiloxanes (P), also contains suitable dye molecules which align along the mesogens and at the same time do not prevent the formation of the liquid-crystalline phase. Optical storage media, which are based on a local change in the refractive index, can be produced by locally modifying the alignment of the mesogenic radicals of the liquid-crystalline organosiloxane (P) before crosslinking. This can be achieved, for example, by local UV crosslinking through a mask which is opaque to UV radiation if the alignment forces acting from the outside or the temperature of the LC layer are modified between the individual exposure steps. Another possibility is structuring of the alignment layer, as used, for example, in LCD manufacture for the production of sub-pixels.
If the liquid-crystalline organosiloxanes (P) or mixtures thereof either with one another or with other liquid-crystalline or non-liquid-crystalline which do not prevent the formation of a liquid-crystalline phase, contain compounds which induce a chiral nematic phase (chiralics), then these can be used for the production of polarizing and wavelength-selective optical filters or LC platelets.
Cholesteric liquid crystals (CLCs) of this type reflect circular-polarized electromagnetic radiation in a wavelength range which depends on the helical structure of the CLC. The chiralics produce either a right-handed or left-handed twisted structure which reflects circular-polarized light of the same helicity. The central wavelength of the reflection band, which is referred to below as the reflection wavelength, is determined by the refractive index and the pitch of the helical structure, which decreases with increasing concentration of the chiralic. In addition, the reflection wavelength is dependent on the viewing angle.
The width of the band is determined by the optical anisotropy of the mesogenic radicals of the liquid-crystalline organosiloxanes (P) and the other mixture components. In most cases, it is between 5% and 15% of the reflection wavelength. For special applications, suitable measures during film production, as described, for example, in U.S. Pat. No. 5,506,704 and U.S. Pat. No. 5,691,789, allow a varying pitch of the helical structure to be produced, which results in an additionally broadened reflection band.
A large number of suitable optically active dopants are known from the literature. For materials with a left-handed helix, it is often possible to rely on cholesterol compounds, which, in addition to chirality, also introduce good mesogenic properties, for example the cholesterol derivatives disclosed by H. Finkelmann, H. Ringsdorf et al., in Makromol. Chem. 179, 829-832 (1978). A suitable steroid system with a right-handed helix based on cholest-8(14)-en-3-ol (doristerol) or derivatives thereof is described in U.S. Pat. No. 5,695,680. Non-steroidal systems tend to reduce the stability of the liquid-crystalline phase at high concentrations. Examples are the tartarimide derivatives disclosed in U.S. Pat. No. 4,996,330 and U.S. Pat. No. 5,502,206. DE-A-43 42 280 and DE-A-44 08 171 describe crosslinkable monomeric hexitol derivatives and mixtures of monomeric dianhydrohexitol derivatives with other liquid-crystalline compounds which are employed as monomeric dopants for the production of cholesteric networks. DE-A-196 19 460 claims liquid-crystal mixtures which contain liquid-crystalline organosiloxanes and dianhydrohexitol derivatives as chiral additives with a left-handed or right-handed helix. The dianhydrohexitol derivatives described therein are preferably compounds from the group consisting of dianhydrosorbide, dianhydromannitol and dianhydroiditol.
CLC mixtures of this type which contain liquid-crystalline organosiloxanes (P) and chiralics can be used for the production, by the process described above, of layers having a cholesteric alignment which reflect circular-polarized light wavelength-selectively. In these applications, the thickness of the LC layer is preferably greater than three times the pitch, up to a maximum layer thickness of 500 xcexcm. Layer thicknesses of from 1 xcexcm to 50 xcexcm are particularly preferred.
Layers of this type having a cholesteric alignment are highly suitable for decorative applications if the concentration of the chiralics is selected so that the reflection wavelength of the cholesteric band is in the visible wavelength region. Owing to the viewing angle-dependent color impression and the metallic sheen, these layers facilitate special color effects. In applications in security paper printing and trademark protection, good copying protection is additionally achieved owing to these color effects and the polarization of the reflected light.
An example of an optical application is a planar CLC filter, as described in U.S. Pat. No. 859,03 1. CLC filters which reflect in the infra-red region (IR) can be employed, for example, for heat-protection glazing. U.S. Pat. No 5,682,212 discloses how wavelength- and polarization-selective elements which are optically imaging for visible light as far as the near ultra-violet (UV) can be produced on curved substrates using cholesteric liquid crystals. Possible use forms of these optical elements are, for example, beam splitters, mirrors and lenses. The liquid-crystalline organosiloxanes (P) are suitable for the production of optical elements of this type from the IR into the UV region, which is accessible, for example, by using the mixtures described in DE-A-196 19 460.
In some applications of the optically anisotropic layers, it is also possible to use a layer containing LC platelets instead of a coherent film. In this way, the special optical effects of the liquid-crystalline organosiloxanes (P) can be applied with significantly less effort, since the user can utilize conventional printing and coating technologies instead of himself carrying out the more complex production of the films, which requires an alignment and crosslinking operation.
To this end, the LC platelets are incorporated into a suitable binder system, as described, for example, in U.S. Pat. No. 5,362,315 and U.S. Pat. No. 5,683,622. The LC platelets containing liquid-crystalline organosiloxanes (P) containing xcex1,xcex2-unsaturated carboxylic acid radicals of high reactivity, such as, for example, acryloyl groups, are particularly suitable for this purpose since, owing to their high crosslinking density, they have improved stability in the binder.
The requisite properties of the binder systems, in particular the optical properties, also depend on the intended application of the LC platelets. For example, in applications which utilize the polarization- and wavelength-selective reflection of LC platelets containing chiral additives, the binders preferably employed are optically transparent at least in the region of the reflection wavelength. For applications which utilize the optical anisotropy in the region of visible light, preferred binders are colorless and transparent throughout the visible wavelength region.
Preferred binder systems for optical elements are those whose mean refractive index after curing is similar to the mean refractive index of the LC platelets. For the production of durable layers containing LC platelets, curable binder systems are preferably suitable. However, non-curable binders, such as, for example, oils, pastes or thermoplastics, can also be used for specific applications.
Particular preference is given to binder systems which do not alter the physical properties of the LC platelets, or only do so in a defined manner. Examples of suitable binder systems are polymerizable resins (UP resins, silicone resins, epoxy resins), dispersions, solvent-containing or water-based coatings, or all transparent plastics, for example polyvinyl chloride, polymethyl methacrylate and polycarbonate. Besides these isotropic binders, the binder used can also be liquid-crystalline systems, for example liquid-crystalline polymers or polymerizable liquid-crystalline resins, and polymerizable LC silicones. In order to produce a layer or a film having specific optical properties, the LC platelets are stirred into a liquid binder. The alignment of the platelets parallel to the surface of the layer is achieved, as in surface coating with liquid-crystalline colored pigments, for example as described in U.S. Pat. No. 5,362,315, on application of a thin layer of the pigment/binder mixture to a substrate or on extrusion of the mixture. Depending on the requirements of the particular application and the properties of the binder, the film can be detached from the substrate after curing.
The applications of the LC platelets produced can, as in the case of the films, be restricted to pure phase retardation of electromagnetic waves from the ultra-violet to the infra-red region or, if a liquid-crystalline organosiloxane containing chiralics is used for the production of the platelets, LC platelets having a liquid-crystalline structure with a chiral phase which reflect electromagnetic waves of a certain wavelength in a circular-polarized manner can also be produced therefrom, as described in U.S. Pat. No. 5,362,315.
An example of an application of LC platelets is the production of optically imaging, wavelength- and polarization-selective elements on curved substrates, as described in U.S. Pat. No. 5,683,622.
The LC platelets are particularly suitable for decorative purposes if the concentration of the chiralics is selected so that the reflection wavelength of the cholesteric band is in the visible wavelength region. In applications in security paper printing and trademark protection, the viewing angle-dependent color impression and the polarization of the reflected light are additional security features. On simultaneous use of LC platelets having a left-handed and right-handed helical structure, prints can be produced, as described in U.S. Pat. No. 5,599,412, which allow the formation of a three-dimensional image on viewing through polarizing spectacles.
For the production of security marks for protection against counterfeiting of, for example, bank notes, security paper prints, documents or in trademark protection, LC platelets can be employed with particular advantage since they can usually be incorporated with relatively little effort into the printing or other coating processes which already exist in these applications. As marks which are invisible to the human eye, IR-reflective LC platelets, which are obtained at low concentrations of chiralics, or UV-reflective LC platelets, which are obtained at high concentrations of chiralics, are particularly suitable since, owing to their good reflection, they can easily be read by instruments having suitable detectors. For such applications, the LC platelets are preferably transparent and colorless in the region of visible light. The wavelength of the reflection band here is preferably above 750 nm or below 400 nm. Besides the reflection wavelength, the circular polarization of the reflected radiation can be detected as an additional security feature. For this application, the CLC platelets are preferably incorporated into an IR- or UV-transparent binder for application to a substrate to be marked.